Then, in January 1939, came the world-shaking discovery of the phenomenon known as uranium fission. In simple language, we had found a proper “match” for lighting a fire with a twin of uranium, the ninety-second, and last, natural element. This twin is a rare form of uranium known as uranium 235—the figure signifying that it is 235 times heavier than common hydrogen. Doubly phenomenal, the discovery of uranium fission meant that to light the atomic fire, with the release of stored-up energy three million times greater than that of coal and twenty million times that of TNT (on an equal-weight basis) would require no match at all. When proper conditions are met, the atomic fire would be lighted automatically by spontaneous combustion.
What are these proper conditions? In the presence of certain chemical agencies, spontaneous combustion will take place when an easily burning substance, such as sawdust, for example, accumulates heat until it reaches the kindling temperature at which it ignites. The chemical agencies here are the equivalent of a match.
The requirement to start the spontaneous combustion of uranium 235, and also of two man-made elements named plutonium and uranium 233 (all three known as fissionable materials or nuclear fuels), is just as simple. In this operation you do not need a critical temperature, but what is known as a critical mass. This simply means that spontaneous combustion of any one of the three atomic fuels takes place as soon as you assemble a lump of a certain weight. The actual critical mass is a top secret. But the noted British physicist, Dr. M. L. E. Oliphant, of radar fame, published in 1946 his own estimate, which places its weight between ten and thirty kilograms. If so, this would mean that a lump of uranium 235 (U-235), plutonium, or U-233, weighing ten or thirty kilograms, as the case may be, would explode automatically by spontaneous combustion and release an explosive force of 20,000 tons of TNT for each kilogram undergoing complete combustion. In the conventional A-bomb a critical mass is assembled in the last split second by a timing mechanism that brings together, let us say, one tenth and nine tenths of a critical mass. The spontaneous combustion that followed such a consummation on August 6 and 9, 1945 destroyed Hiroshima and Nagasaki.
Thus, if we substitute the familiar phrase “spontaneous combustion” for the less familiar word “fission,” we get a clear understanding of what is known in scientific jargon as the “fission process,” a “self-multiplying chain reaction with neutrons,” and similar technical mumbo-jumbo. These terms simply mean the lighting of an atomic fire and the release of great amounts of the energy stored in the nuclei of U-235 since the beginning of the universe. The two so-called man-made elements are not really created. They are merely transformed out of two natural heavy elements in such a way that their stored energy is liberated by the process of spontaneous combustion.
Why, one may ask, does not spontaneous combustion of U-235 take place in nature? Why, indeed, has not all the U-235 in nature caught fire automatically long ago? To this also there is a simple answer. Just as in the spontaneous combustion of sawdust the material must be dry enough to burn, so must the U-235. Only in place of the word “dry” we must use the word “concentrated.” The U-235 found in nature is very much diluted with another element that makes it “wet.” It therefore must be separated first, by a very laborious and costly process, from the nonfissionable, or “wetting,” element. Even then it won’t catch fire, and could not be made to burn by any means, until the amount separated (“dried”) reaches the critical mass. When these two conditions—conditions that do not exist in nature—are met, the U-235 catches fire just as sawdust does when it reaches the critical temperature.
The fact that as soon as a critical mass is assembled the three elemental atomic fuels burst into flame automatically thus puts a definite limit to the amount of material that can be used in the conventional A-bomb. The best you can do is to incorporate into a bomb two fragments, let us say, of nine tenths of a critical mass each. To enclose more than two such fragments would present difficulties that appear impossible to overcome. It is this limitation of size, an insurmountable roadblock put there by mother nature, that makes the basic difference between the A-bomb and the H-bomb.
For, as we have already seen, to light an atomic fire with deuterium it is necessary to strike a match generating a flame with a temperature of about 50,000,000 degrees centigrade. As long as no such match is applied, no fire can start. It thus becomes obvious that deuterium is not limited by nature to a critical mass. A quantity of deuterium a thousand times the amount of the U-235, and hence a thousand times more powerful, can therefore be incorporated in an ordinary A-bomb, where it would remain quiescent until the A-bomb match is struck. Weight for weight, deuterium has only a little more energy content than U-235, so that a bomb incorporating a 1,000 kilograms (one ton) of deuterium would thus have an energy of 20,000,000 tons of TNT.
Here must be mentioned another form of hydrogen, named tritium. It has long ago disappeared from nature but it is now being re-created in ponderable amounts in our atomic furnaces. Tritium, the nucleus of which is known as a triton, weighs three times as much as the lightest form of hydrogen. It has an energy content nearly twice that of deuterium. But it is very difficult to make and is extremely expensive. Its cost per kilogram at present AEC prices is close to a billion dollars, as compared with no more than $4,500 for a kilogram of deuterium. A combination of deuterons and tritons would release the greatest energy of all, 3.5 times the energy of deuterons alone. It would reduce the amount of tritons required to half the volume and three fifths of the weight required in a pure triton bomb, thus making the cost considerably lower.
But why bother with such fantastically costly tritons when we can get all the deuterium we want at no more than $4,500 a kilogram, while we can make up the difference in energy by merely incorporating two to three and a half times as much deuterium? Here we are dealing with what is probably the most ticklish question in the design of the H-bomb.
To light a fire successfully, it is not enough merely to have a match. The match must burn for a time long enough for its flame to act. If you try to light a cigarette in a strong wind, the wind may blow out your match so fast that your cigarette will not light. The same question presents itself here, but on a much greater scale. The match for lighting deuterium—namely, the A-bomb—burns only for about a hundred billionths of a second. Is this time long enough to light the “cigarette” with this one and only “match”?